This invention relates to a control method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind which can improve the engine driveability and emission characteristics when the engine is operating in a low load operating region wherein feedback control should be interruped.
An air-fuel ratio control method for an internal combustion engine is generally known wherein the fuel quantity supplied to the engine is controlled in a feedback manner responsive to an output signal from an exahust gas sensor which detects the concentration of an exhaust gas ingredient, in order that the air-fuel ratio of the mixture supplied to the engine becomes equal to a desired ratio (e.g. the stoichiometric mixture ratio).
It is also known e.g. from Japanese Provisional Patent Publication (Kokai) No. 59-539 to provide mixture-leaning regions which are defined by engine operation parameters (e.g. vehicle speed, engine coolant temperature, intake pipe absolute pressure, engine rotational speed), and control the air-fuel ratio of the mixture supplied to the engine to a value larger or leaner than the stoichiometric mixture ratio while interrupting the feedback control when the engine is operating in any of the mixture-leaning regions, to thereby reduce the fuel consumption. The leaning of the mixture is effected by multiplying a basic value of fuel supply quantity determined by intake pipe absolute pressure, engine rotational speed, etc. by a mixture-leaning correction coefficient having a fixed value.
The mixture-leaning regions usually include a low-load high speed operation region. In this region, if the basic value of fuel supply quantity is multiplied by a fixed mixture-leaning coefficient as is done in the conventional method, there is a fear that the air-fuel ratio becomes excessively leaner than the desired lean value or excessively richer (about 16) than the latter when there is a deviation of the basic value of fuel supply quantity from a proper value, which results in such problems as poor driveability due to engine output shortage, or high NOx concentration in the exhaust gases.
Japanese Provisional Patent Publication No. 59-539 also discloses that when the engine coolant temperature as the engine temperature is lower than a predetermined value the engine operating region wherein mixture-leaning is effected is made narrower so as to avoid degradation of the emission characteristics as well as degradation of the driveability due to mixture-leaning. However, at low ambient temperature and hence at low engine intake air temperature, injected fuel is not atomized to a sufficient degree, which results in poor combustion of the mixture even if the engine coolant temperature is high, and consequent emission of large amounts of CO and HC. If the mixture is leaned under this poor combustion condition the engine output will be too low to obtain required vehicle driveability. Any conventional methods have been unable to solve these problems.